The invention relates to an electronic control device for an electric motor, e.g., an electronically commutated motor, whose motor windings are connected in series with respective control transistors to control the winding currents, and with the control connections of the transistors being charged by control pulses triggered by commutation signals and having a predeterminable pulse-duty factor.
Various alternatives are available for controlling electric motors. For example, an electronically commutated motor (EC motor) is known that has a high-frequency cadence. This type actuation, however, is not applicable to low-power motors, such as servomotors in motor vehicles, because of the considerable circuit complexity and problems with radio interference which are very difficult to overcome.
Moreover, an EC motor is known that has linear actuation. In this case, the rpm is set by operating the power transistors connected in the circuits of the motor windings in their amplifying range. The transistor-resistance additionally functioning in the armature circuit causes the characteristic curve of the motor to dip around the idle point. This process essentially corresponds to a series resistor control. The output power in this case is proportional to the third power of the rpm, whereas the received power is proportional to the square of the rpm. Consequently, at 2/3 of the nominal output, the stray power is the highest, and efficiency at this rpm is 66% of the maximum possible efficiency. Linear control thus inevitably leads to a reduced efficiency proportional to the rpm in the partial control range. In addition to the low efficiency, a high temperature stress of the power electronics occurs during partial load operation, because the lost power is converted into heat.
Finally, block control is also known. In this instance, the rpm is set by means of the change in the actually used angle of current flow with respect to the commutation angle. The control transistors are controlled so that they are either completely switched through (conducting) or completely switched off (non-conducting); however, the turn-on or energization time of the control transistors is varied. In this case the angle of current flow can have a fixed reference at the initiation of the commutation time. However, the deenergization point (end of the commutation angle) can be maintained while the energization time is varied. Moreover, it is possible to vary both the energization and deenergization times symmetrically to the mid-point of the commutation angle. In each case the transistors are operated in the switching mode, that is, with minimum volume resistance in MOSFETs, or with minimum collector-emitter voltage in bipolar transistors. The mode of operation is similar to a pulse-width-modulated low set point regulator. It is disadvantageous that at low motor rpm and subsequently at low induced voltage, the current can attain very high amplitude values during the current flow time. The connection or relationship can nearly be recognized from the following equation: ##EQU1## The current-driving voltage (the difference of the battery or supply voltage U.sub.B and the induced voltage u.sub.i of the motor) is applied to a series connection of the winding inductance L and the winding resistor R. Consequently, the current amplitude, which is limited by resistor R, and results from the induced voltage, and thus the rpm, is ##EQU2## Only the changes in current di/dt are inhibited by the inductance L of the motor.
As a result, when the motor is rotating slowly (u.sub.i is small, f has a low frequency), very high peak loads occur that in turn cause the development of intense noise by means of instantaneous pulsation in direct association with the torque. Moreover, a strongly pulsing current draw has a negative effect on the overall supply mains, for example, in a vehicle.
It is therefore an object of the present invention to provide an improved control device which avoids disadvantages of the above mentioned type control devices.